It is known to deposit soft overlay alloy compositions onto the running surface of a bearing lining by cathodic sputtering. An example of this is described in U.S. Pat. No. 5,045,405, in which an overlay based on a matrix of copper, aluminium or silver is described having columnar crystal grains with a major axis normal to the sliding direction and an embedded phase of particles which are softer than the matrix and also insoluble in the matrix metal. Examples are given of overlay compositions of AlSn20 (80% weight aluminium matrix with 20% wt of tin) and AlPb30 (70% weight aluminium matrix with 30% weight of lead).
Such two-phase composite (suspension) alloys had previously been known to be used as wrought or cast materials, as well as having been proposed for deposition by a plasma arc spraying process, as described in UK patent application GB2130250.
The use of such two-phase alloys as the overlay is attributable to the search for materials having both improved wear resistance and improved fatigue strength over conventional electro-deposited lead and tin based alloys whilst retaining good conformability (ability to accept the embedding of waste particles into the overlay) and anti-seizure properties.
However, in the initial stages of running of a new shaft in an engine, it is desirable to have a higher degree of conformability than is readily provided from known two-phase alloys, such as is described in U.S. Pat. No. 5,045,405. Furthermore, as the overlay wears and the running clearance between shaft journal and bearing increases it is desirable to provide an increasing degree of wear resistance in the overlay material, in contrast to the substantially uniform properties of U.S. Pat. No. 5,045,405. With increasing overlay wear, conformability becomes less important and wear resistance becomes more important.
European patent EP0435980 concerns an overlay with good conformability at the surface and increasing wear resistance away from the surface. It discloses a journal bearing including a strong backing material having thereon a bearing material and an overlay coating on said layer of bearing material, the overlay coating comprising a first material constituting the coating matrix and having therein a dispersed phase of a second material that is substantially insoluble in the first material, the overlay coating being deposited by cathodic sputtering. The dispersed phase of the second material has a relatively low content at the interface between the layer of bearing material and the coating, and said content continuously varying towards a relatively high content at the coating surface, the total thickness of said overlay coating being in the range from 10 to 50 μm.
EP0435980 discloses an exemplary embodiment of an overlay coating comprising: an aluminium-tin composite material having a first layer of substantially pure aluminium and a thickness of approximately 2 μm; adjacent the first layer is an intermediate layer having an aluminium matrix with dispersed particles of tin therein, in which the intermediate layer has a composition of approximately aluminium with 10 wt % tin adjacent the first layer and incrementally rising steadily in tin content to approximately 50 wt % tin at a thickness of approximately 22 μm; the coating has, as a surface layer, a layer of substantially pure tin approximately 2 μm thick.
It is known that increasing the proportion of soft-phase (e.g. the proportion of tin to aluminium) in the matrix towards the surface of the coating increases the gain size of the soft-phase dispersed in the matrix, which reduces the hardness and increases the conformability of the coating towards the surface (away from the bearing backing). However, disadvantageously the manufacture of such a bearing lining requires sophisticated deposition apparatus operating complex manufacturing procedures in which the proportion of soft-phase deposited requires to be accurately controlled in real-time during the deposition process, with adverse implications for cost, manufacturing reliability and yield, and the manufacturing throughput of each deposition apparatus.
Alternatively, it is also known to increase the grain size of tin particles suspended within an aluminium-based matrix, over the course of the coating deposition, by increasing the temperature of the bearing backing during the sputter deposition, as is disclosed in U.S. Pat. No. 4,916,026. However, disadvantageously, the manufacture of such a bearing lining requires sophisticated deposition apparatus operating complex manufacturing procedures in which the temperature requires to be accurately controlled in real-time during the deposition process, with adverse implications for cost, manufacturing reliability and yield, and the manufacturing throughput of each deposition apparatus. Further, disadvantageously, differences in heat capacity will affect the required temperature control of the bearing lining during the deposition process so that different deposition processes will be required for different sizes of bearing linings, and even for nominally identical bearing linings using the same deposition process there will be variations in the characteristics of the deposited coatings due to the manufacturing tolerances of the bearing lining.
A particular challenge to bearing lining performance is provided by the configuration of vehicle engines to fuel-saving stop-start operation, in which the engine is stopped each time the vehicle stops, on contrast to conventional operation in which the engine is kept running throughout a vehicle's journey. Engines configured for stop-start operation may typically restart their engines about one hundred times more frequently than conventionally configured engines run continuously throughout each vehicle journey. The particular problem that an engine configured for stop-start operation presents arises because engine bearings are conventionally hydrodynamically lubricated, with little or no lubrication initially being provided to the bearing when it starts, leading to particularly significant wear during the start-up phase.
Accordingly, a need remains in the art for an improved bearing lining coating and a corresponding manufacturing process.